CHAP. IV.] 



THE TELESCOPE. 



273 



under favourable circumstances gave the best results was placed 

 in the first rank and put aside, and the nine others were worked at. 

 When one of these became better than the first, it took its place, 

 until, in turn, another took the lead, and so on. It is curious to 

 learn on what a large scale these experiments were carried on even 

 at a time when Herschel was only a simple amateur astronomer in 

 the city of Bath. He made as many as 200 Newtonian mirrors of 

 7 feet focus (213m), 150 mirrors of 10 feet focus (3*05m.), and about 

 24 mirrors of 20 feet (6'096m). 



" ' Each time Herschel undertook to polish the mirror of a telescope,' 

 says Lalande, ' he worked continuously for ten, twelve, arid fourteen 

 hours. He never left it an instant, not even to eat, and received 

 from his sister's hand the nourishment without which it would have 



Fro. 206. Principle and arrangement of Newton's telescope. 



been impossible to undergo such long fatigue. Herschel would not 

 leave his work for any consideration ; according to him, this would 

 have spoilt it.' " 



Telescopes with metal mirrors have serious inconveniences ; 

 besides the enormous weight of the mirror, when the aperture 

 is considerable, they have the defect of requiring frequent polishing, 

 as they tarnish under the influence of atmospheric moisture. The 

 polishing itself is a delicate operation, as it may change the curve 

 of the mirror. 



Foucault succeeded in diminishing considerably with equal 

 apertures the weight of the mirror, and rendering the curvature 

 nearly unchangeable great advantages in addition to the absence 

 pf chromatic aberration. 



T 



